In applications where reliability at high temperatures is required, metal structures have their limits. Ceramics, unlike metals, can maintain their reliability above the temperature limit of metals. If ceramics could be made into complex shapes and large sizes, with the relative ease that metal structures can be made, then ceramics could replace metals in applications that would benefit from the higher temperatures that ceramics can achieve.
Making ceramics into complex shapes and large sizes is difficult. Ceramics are hard and brittle, so that where metals can be regularly machined into complex shapes and formed in large sizes; ceramics are normally made in simple shapes and small sizes to avoid the difficulties of machining and forming in ceramics. However, if ceramics were joined, then the simple shapes and small sizes of produced ceramics could be joined together to achieve the complex shapes and large sizes that metals can have.
Joining ceramics that are reliable at high temperatures has difficulties. Ceramics can be diffusion bonded, a method of reaching a processing temperature close to the melting point of the ceramic so that both sides of the joint diffuse into one another. However, it is expensive to reach temperatures close to the melting point of ceramics and the ceramic can become weak or slag at these extreme temperatures. Joining with metal braze imparts metallic impurity into the joint that may corrode and contaminate. Also, the mismatch in coefficient of thermal expansion causes high stress in the joint at high temperatures. Using preceramic polymers to form a joint between two ceramics can be accomplished at temperatures comparable to the application-specific service temperature of the ceramic and converts to ceramic, so there are no issues of thermal mismatch and contamination.
For silicon carbide, preceramic polymers such as allylhydridopolycarbosilane, polyborosiloxane, and polysilazane are used to join silicon carbide together. The polymers, however, exhibit shrinkage during conversion of the polymer to ceramic. This brings high stresses to the joint and the joint becomes very porous and large cracks are formed. To make an effective joint, multiple infiltrations of the polymer can form a denser interlayer. For most preceramic polymers, 8-10 infiltrations is normally required to produce a strong joint. Because infiltrations are time-consuming, filler materials can be used to reduce the amount of infiltrations to produce a strong joint. Filler materials, such as silicon carbide powder, can be used to reduce the volume shrinkage of the polymer during conversion. However, filler materials can be ineffective since they become stationary inside the polymer before conversion into ceramic, thus rendering them inactive in filling pores when the polymer experiences shrinkage later in processing.
The thickness of the preceramic polymer will determine the strength of the joint as well. Therefore, tight tolerances are normally held in the joint components and even surface roughness can affect the effectiveness of the joint. Heavy machining and mirror polished flats are made to reduce the thickness of the joint material to create a strong joint.